Electromagnetic (EM) geophysical exploration systems measure the response of subsurface formations to the propagation of naturally or artificially generated EM fields. Primary EM fields may be generated by passing alternating current or pulsing a current through a transmitter coil, which is an electrically conducting wire or tube that may have an air core or be wrapped around a core made of some electrical conductor. Use of an alternating current is referred to as frequency domain EM while the use of a pulsed current where the current is applied during an on-period and switched off during an off-period is referred to as time-domain EM or transient EM. In both cases, the time-variation of current passing through the transmitter coil produces a magnetic field in a large vicinity around the transmitter coil. A transmitter coil may be a small coil made up of many turns of wire or a large loop of wire with one or more turns. Subsurface formations respond to the propagation of time-varying primary EM fields with the generation of secondary electrical currents by the process of EM induction (which is the production of a voltage across a conductor when it is exposed to a time-varying magnetic field) giving rise to secondary EM fields. The primary and secondary EM fields may be detected by a “receiver.” A receiver may measure the time-variation of the magnetic field from these currents (for example a coil receiver measuring dB/dt) or may measure the magnetic field itself (a B-field sensor). Hereinafter, the terms “transmitter coil,” “transmitter loop,” and “transmitter” may be used interchangeably and the terms “receiver coil,” “receiver loop,” and “receiver” may be used interchangeably.
The primary EM field propagates from the transmitter coil to the receiver via paths both above and below the surface of the earth. In the presence of a conducting body or earth material such as soils, rocks, ores or other conducting material, the magnetic component of the EM field penetrating the subsurface induces time-varying currents, or eddy currents, to flow in the conducting body. The eddy currents generate their own EM field (referred to as secondary EM field) that travels to the receiver. The receiver then undergoes a response to the resultant of the arriving primary and secondary EM fields so that the response differs in both phase and amplitude from the response to the primary EM field alone. Differences between the transmitted and received EM fields reveal the presence of the conducting body or conducting material and provide information on the conducting body's geometry and electrical properties.
Because EM fields propagate through air, there is no need for physical contact of either the transmitter coil or receiver coil with the earth's surface. EM geophysical exploration can thus proceed much more rapidly than galvanic method surveys, where ground contact is required. More importantly, one or both of transmitter coil and receiver coil can be mounted in or on or towed behind aircraft. Airborne EM methods are used in prospecting for conductive ore bodies and many other geological targets due to their speed and relative cost-effectiveness.
The EM response from subsurface materials or bodies is dependent on the electrical conductivity of the material or body. Ore bodies or other structures such as layers that have low electrical conductivity may still provide an EM response.
Thus, in summary, EM surveying or geophysical exploration uses the principle of EM induction to measure the electrical conductivity of the subsurface. In the case of a frequency-domain EM survey, an alternating electric current of known frequency and magnitude is passed through a transmitter coil creating a primary EM field in the space surrounding the coil, including underground. The time-varying EM fields induce a secondary current in underground conductors or structures which results in an alternating secondary magnetic field that is sensed by the receiving coil. The secondary field is distinguished from the primary field by a phase lag. The ratio of the magnitudes of the primary and secondary currents is proportional to the terrain conductivity. The depth of penetration of the EM field into the subsurface is governed by the subsurface electrical conductivity and transmitter excitation frequency and coil separation and orientation.
In the case of a transient EM survey, the same principle of EM induction is used to measure the electrical conductivity of the subsurface. A pulsed electric current of known amplitude and time-occurrence is passed through a transmitter coil creating a primary EM field in the space surrounding the coil, including underground. The eddy currents generated in the ground in turn induce a time-varying secondary magnetic field that is sensed by the receiving coil. In the off-time of the transmitter, the signal magnitude and time-variation of the signal magnitude is proportional to the terrain conductivity. In the on-time of the transmitter, the received signal is proportional to the terrain conductivity and to the transmitted primary signal. The depth of penetration of the EM field into the subsurface is governed by the terrain conductivity, transmitter power, transmitter excitation frequency and coil orientation.
In time-domain EM systems, the voltage in the receiver is proportional to the time rate of change of the current in the transmitter loop. The transmitter of these systems is designed to generate a magnetic impulse and then turn off the primary field so that the secondary fields from currents induced in the subsurface can be detected. Practically, it is very difficult to produce an ideal impulse because when an electric circuit is switched off, the electrical limitations of the system (for example, self-inductance) result in some residual current flow for a time. These remaining currents are detected by the receiver, thereby interfering with detection of the secondary fields from currents induced in the subsurface. Thus, a need has arisen for systems and methods that address these shortcomings of traditional EM geophysical exploration systems and methods.